zfs: reject read(2) of a dirfd with EISDIR

This is independent of the recently-discussed global change, which is still
in review/discussion stage.

This is effectively a measure for consistency in the ZFS world, where
FreeBSD was the only platform (as far as I could find) that allowed this.
What ZFS exposes is decidedly not useful for any real purposes, to
paraphrase (hopefully faithfully) jhb's findings when exploring this:

The size of a directory in ZFS is the number of directory entries within.
When reading a directory, you would instead get the leading part of its raw
contents; the amount you get being dictated by the "size," i.e. number of
directory entries. There's decidedly (luckily) no stack disclosure happening
here, though the behavior is bizarre and almost certainly a historical
accident.

This change has already been upstreamed to OpenZFS.

MFC after: 1 week